Due to the prodigious quantities of thermal energy produced by fissioning nuclear fuel, it is imperative to maintain the fuel core of commercial water cooled nuclear fission reactors submerged within heat transferring coolant water. The conveyance of heat out away from the energy producing fuel core by means of circulating coolant water is needed to preclude the possibility of hazardous conditions or reactor damage such as could occur with an overheating meltdown within the fuel core of the reactor plant.
Such a potentially destructive occurrence can result from a loss of coolant accident (LOCA) caused by an extensive breach of a major reactor coolant receptacle or conduit. To cope with this theoretical accidental event, commercial water cooled nuclear fission reactors are provided with large reservoirs of water available for supplying supplementary coolant water to the reactor vessel for cooling the fuel core and maintaining lower or normal operating temperatures. A variety of safety measures have been proposed or employed to activate and operate systems for supplying or injecting this supplementary coolant water as needed to the fuel core for replacing or supplementing any loss of the original coolant water due to some mishap.
A typical arrangement in commercial water cooled nuclear fission reactor plants for incorporating standby safety systems which feed or inject auxiliary coolant water to temper the fuel core temperatures utilize an apt gas, such as nitrogen, for a propellant to drive the supplementary liquid water or a boron solution from a source or reservoir through communicating conduits into the reactor vessel. Thus, auxiliary coolant water or an aqueous boron solution is maintained within a closed vessel or tank under sufficient gas pressure to drive the liquid contents into the reactor vessel through an appropriate arrangement of conduits upon a manually or automatically actuated signal responding to a malfunction within the reactor.
However, such systems are prone to leakage and loss of gas for propelling coolant water as well as prone to malfunctioning of the manual and/or mechanical or electronic means for actuation of the system.
Another means comprises gravity feed arrangements employing elevated vessels of auxiliary coolant water. However loss-of-coolant accidental events can sometimes result in overheating which in certain cases causes increased pressures above the already high pressures within the reactor pressure vessel. The occurrence of such elevated pressure conditions inhibits gravity feeding of coolant water into a highly pressurized reactor vessel.